A roller cone rock bit is a commonly used cutting tool used in oil, gas, and mining fields for breaking through earth formations and shaping well bores. Reference is made to FIG. 1 which illustrates a cross-sectional view of a portion of a typical roller cone rock bit. FIG. 1 specifically illustrates the portion comprising one head and cone assembly of the bit. The general configuration and operation of such a bit is well known to those skilled in the art.
The head 10 of the bit includes a downwardly and inwardly extending bearing shaft 12. A cutting cone 14 is rotatably mounted on the bearing shaft 12. The bearing system for the head and cone assembly that is used in roller cone rock bits to rotatably support the cone 14 on the bearing shaft 12 typically employs either rollers as the load carrying element (a roller bearing system) or a journal as the load carrying element (a friction bearing system). FIG. 1 specifically illustrates a roller bearing implementation including a bearing system defined by a first roller bearing 16 (also referred to as the main roller bearing). The cone 14 is axially retained on the bearing shaft 12, and further supported for rotation, by a set of ball bearings 18 that ride in the annular raceway 20 defined at an interface between the bearing shaft 12 and cone 14. The ball bearings 18 are delivered to the raceway 20 through a ball opening 46, with that opening 46 being closed by a ball plug 48. The ball plug 48 is shaped to define a portion of the lubricant channels 28 within the ball opening 46. The ball bearing system as shown would typically also be present in bearing system implementations which utilize friction journal bearings. The bearing system for the head and cone assembly further includes second roller bearing 22, first radial friction (thrust) bearing 24 and second radial friction (thrust) bearing 26.
The bearing system for the head and cone assembly of the bit is lubricated and sealed. The interstitial volume within the bearing system defined between the cone 14 and the bearing shaft 12 is filled with a lubricant (typically, grease). This lubricant is provided to the interstitial volume through a series of lubricant channels 28. A pressure compensator 30, usually including an elastomer diaphragm, is coupled in fluid communication with the series of lubricant channels 28. The lubricant is retained within the bearing system by a sealing system 32 provided between the base of the cone 14 and the base of the bearing shaft 12. The configuration and operation of the lubrication and sealing systems within roller cone drill bits are well known to those skilled in the art.
A body portion 34 of the bit, from which the head and cone assembly depends, includes an upper threaded portion forming a tool joint connection which facilitates connection of the bit to a drill string (not shown, but well understood by those skilled in the art).
FIG. 2 illustrates a cross-sectional view of the bit shown in FIG. 1 focusing on a bearing shaft and cone in greater detail. The first roller bearing (main roller bearing) 16 is defined by an outer cylindrical surface 40 on the bearing shaft 12 and a set of roller bearings 42 provided within an annular roller raceway 44 in the cone 14. In a friction journal bearing system, the outer cylindrical surface 40 on the bearing shaft 12 would interact with an inner cylindrical surface of the cone 14 or a bushing (a ring-shaped structure typically made of beryllium copper) that is press fit into an annular aperture formed in the cone 14.
As discussed above, lubricant is retained within the bearing system by a sealing system 32. The sealing system 32, in a basic configuration, comprises an o-ring type seal member 50 positioned in a seal gland 52 between the cutter cone 14 and the bearing shaft 12 to retain lubricant and exclude external debris. A cylindrical sealing surface 54 is provided at the base of the bearing shaft 12. The annular seal gland 52 is formed in the base of the cone 14. The gland 52 and sealing surface 54 align with each other when the cutting cone 14 is rotatably positioned on the bearing shaft 12. The o-ring sealing member 50 is compressed between the surface(s) of the gland 52 and the sealing surface 54, and functions to retain lubricant within the bearing system. This sealing member 50 also prevents materials from the well bore (such as drilling mud and debris) from entering into the bearing system.
Over time, the rock bit industry has moved from a standard nitrile material for the seal member 50, to a highly saturated nitrile elastomer for added stability of properties (thermal resistance, chemical resistance). The use of a sealing system 32 in rock bit bearings has dramatically increased bearing life in the past fifty years. The longer the sealing system 32 functions to retain lubricant within the interstitial volume, and exclude contamination of the bearing system, the longer the life of the bearing and drill bit. The sealing system 32 is, thus, a critical component of the rock bit.
The second roller bearing 22 of the bearing system is defined by an inner cylindrical surface 60 on the cone 14 and a set of roller bearings 62 provided within an annular roller raceway 64 in the bearing shaft 12. The first radial friction (thrust) bearing 24 of the bearing system is defined between the first and second cylindrical friction bearings 16 and 22 by a first radial surface 66 on the bearing shaft 12 and a second radial surface 68 on the cone 14. The second radial friction (thrust) bearing 26 of the bearing system is adjacent the second roller bearing 22 at the axis of rotation for the cone and is defined by a third radial surface 70 on the bearing shaft 12 and a fourth radial surface 72 on the cone 14.
The lubricant is provided in the interstitial volume that is defined between the surface 40 and raceway 44 of the first roller bearing 16, the surface 60 and raceway 64 of the second roller bearing 22, the surfaces 66 and 68 of the first radial friction bearing 24 and the surfaces 70 and 72 of the second radial friction bearing 26. The sealing system 32 with the o-ring type seal member 50 positioned in the seal gland 52 functions to retain the lubricant within the lubrication system and specifically between the opposed surfaces of the bearing system.
During operation of the bit, the rotating cone 14 oscillates along the head in at least an axial manner. This motion is commonly referred to in the art as a “cone pump.” Cone pumping is an inherent motion resulting from the external force that is imposed on the cone by the rocks during the drilling process. The oscillating frequency of this cone pump motion with respect to the head is related to the rotating speed of the bit. The magnitude of the oscillating cone pump motion is related to the manufacturing clearances provided within the bearing system (more specifically, the manufacturing clearances between the surface 40 and raceway 44 of the first roller bearing 16, the surface 60 and raceway 64 of the second roller bearing 22, the surfaces 66 and 68 of the first radial friction bearing 24 and the surfaces 70 and 72 of the second radial friction bearing 26). The magnitude is further influenced by the geometry and tolerances associated with the retaining system for the cone (for example, the ball race). When cone pump motion occurs, the interstitial volume defined between the foregoing cylindrical and radial surfaces of the bearing system changes. This change in volume squeezes the lubricant provided within the interstitial volume.
The change in interstitial volume and squeezing of the lubricant grease results in the generation of a lubricant pressure pulse (that pulse generally originating at or near the radial thrust bearings). Over a very short period of time, responsive to this pressure pulse, grease flows along a first path 74 between the bearing system and the pressure compensator 30 through the series of lubricant channels 28 (see, also FIG. 1). The pressure compensator 30 is designed to relieve or dampen the pressure pulse by compensating for volume changes through its elastomer diaphragm. However, it is known in the art that the pressure pulse, notwithstanding the presence and actuation of the pressure compensator 30, can also be felt at the sealing system 32 due to the presence of other paths for the flow of grease, responsive to this pressure pulse, between the opposed surfaces of the bearing system and the sealing system 32. For example, grease may flow along a second path 76 through the raceway 20 and along the surface 40 between the bearing system and the sealing system 32. Additionally, grease may flow along a third path 78 between the bearing system and the sealing system 32 through the raceway 20, the series of lubricant channels 28 and along the surface 40. Still further, grease may flow along a fourth path 80 between the bearing system and the sealing system 32 through the raceway 64, the series of lubricant channels 28 and along the surface 40.
The flow of grease along the second through fourth (and perhaps other) paths in response to the pressure pulse is known to be detrimental to seal operation and can also reduce seal life. For example, positive and negative pressure pulses due to cone pump motion may cause movement of the sealing member 50 within the seal gland. A nibbling and wearing of the seal member 50 may result from this movement. Additionally, a positive pressure pulse due to cone pump motion may cause lubricant grease to leak out past the sealing system 32. A negative pressure pulse due to cone pump motion may pull materials in the well bore (such as drilling mud and debris) past the sealing system 32 and into the bearing system.
Reference is now made to FIG. 3 which shows a cross-section of a portion of a bit with a cylindrical friction bearing (also referred to as the main journal bearing). The cylindrical friction bearing 16′ is defined by an outer cylindrical surface 40′ on the bearing shaft 12 and an inner cylindrical surface 42′ of a bushing 44′ which has been press fit into the cone 14. This bushing 44′ is a ring-shaped structure typically made of beryllium copper, although the use of other materials is known in the art. Ball bearings 18 ride in an annular raceway 20 defined at an interface between the bearing shaft 12 and cone 14. The ball bearings 18 are delivered to the raceway 20 through a ball opening 46, with that opening 46 being closed by a ball plug 48. The ball plug 48 is shaped to define a portion of the lubricant channels 28 within the ball opening 46.
Cone pumping is also a concern in bits using the cylindrical friction bearing 16′. Again, responsive to this pressure pulse (generally originating at or near the radial thrust bearings), grease flows along a first path 74 between the bearing system (for example, the thrust bearing surfaces) and the pressure compensator 30 through the series of lubricant channels 28 (see, also FIG. 1). The pressure compensator 30 is designed to relieve or dampen the pressure pulse by compensating for volume changes through its elastomer diaphragm. However, it is known in the art that the pressure pulse, notwithstanding the presence and actuation of the pressure compensator 30, can also be felt at the sealing system 32 due to the presence of other paths for the flow of grease. For example, grease may flow along a second path 76 through the raceway 20 and along the surface 40′ between the bearing system and the sealing system 32. Additionally, grease may flow along a third path 78 between the bearing system and the sealing system 32 through the raceway 20, the series of lubricant channels 28 and along the surface 40′.
As discussed above, this pressure pulse may have detrimental effects on the sealing system 32 and particularly the sealing member 50. There is accordingly a need in the art to reduce, or eliminate, the pressure pulsation due to cone pumping from acting on the sealing system 32.